Expansible tube valve with low pressure bleed

ABSTRACT

A pressure regulating system including a main valve with an expansible tube stretched over a cylindrical barrier so that flow occurs around the barrier only when a pressure fluid in a control chamber around the tube is overcome by upstream pressure. A pilot valve senses downstream pressure and opens when it drops to a predetermined level to exhaust the control chamber to a low pressure zone. Flow around the barrier is increased by evacuating the control chamber to a zone within the valve housing at a pressure lower than downstream. This occurs in a zone wherein the static pressure of the downstream fluid line is converted by the flowing stream to a high velocity, low static pressure. In areas where an atmospheric bleed is not objectionable, a second pilot valve may be provided to exhaust the control chamber to atmosphere if downstream pressure drops to a set lower level.

Gerbic [54] EXPANSIBLE TUBE VALVE WITH LOW PRESSURE BLEED 1 June 13,1972 215,348 10/1957 Australia ..l37/489 Primary Examiner-M. Cary Nelson[72] Inventor: Charles C. Gerbic, San Anselmo, Calif. Assismm Emmm" RbenJ. Miller [73] Assignees Grove Valve and Regulator Company,Anomey-Melvi" Smdham Oakland, Calif.

ABSTRACT [22] Filed: Oct. 16, 1970 A pressure regulating systemincluding a main valve with an PP 31,311 expansible tube stretched overa cylindrical barrier so that flow occurs around the barrier only when apressure fluid in a s2 u.s.c|.... ..137/439, 251/5, 251/24 COMO! chamberaround the tube is Overcome by upstream 51] 1m. (:1. ..G05d 16/16 P APilot valve senses downstream pressurc and opens 58 Field of Search137/489, 500, 503; 25l/6l.1, when it drops to a predetermined level toexhaust the control 2 5 42 4 4 2 25 chamber to a low pressure zone. Flowaround the barrier is increased by evacuating the control chamber to azone within [56] R f r Cited the valve housing at a pressure lower thandownstream. This occurs in a zone wherein the static pressure of thedownstream UNITED STATES PATENTS fluid line is converted by the flowingstream to a high velocity, low static pressure. In areas where anatmospheric bleed is not g g; g objectionable, a second pilot valve :maybe provided to exry haust the control chamber to atmosphere ifdownstream pres- FOREIGN PATENTS OR APPLICATIONS Sure drops to a setlower level- 255,794 l/ l 949 Switzerland ..25 H24 15 Claims, 2 Drawing;figures 7a as so as 444 l 5/ 4a 47 5e a V 5% A v :2: 6 2a a 22: P2 30 72:2 $2 go- 70 M 24 42 ll 23 24 49 z/ r /0 X 40 44 I7 PATENTEUJUN 13 I9723.669.142

sum 1 [IF 2 INVENTOR CH YRL 55 C. GFRB/C BY ATTORNEY EXPANSIBLE TUBEVALVE WITH LOW PRESSURE BLEED BACKGROUND OF THE INVENTION This inventionrelates to a pressure regulating system and more particularly, to asystem including an expansible tube type valve with a surrounding jacketfor a control pressure fluid which is evacuated by the opening of apilot valve in response to downstream pressure deterioration.

Expansible tube type valves have been employed with considerable successin pressure regulating systems. The expansible tube is carried on acylindrical core sleeve which has two axially spaced inlet and outletseries of through openings around it with an intermediate barrierblocking the flow axially through the core sleeve. A control pressurefluid is in a jacket around the tube normally to prevent expansion ofthe tube away from the core sleeve barrier. The control pressure fluidmay be introduced from the upstream line so that pressures inside andoutside of the tube are balanced. In such case, the tension in the tubestretched over the core sleeve maintains the tube sealed around thebarrier. In a duct connecting the jacket to a low pressure zone is apilot valve which is sensitive to downstream pressure. Hence, if thedownstream pressure falls below a predetermined level, the jacket isbled to the low pressure zone enabling upstream pressure within the tubeto overcome the now-lower jacket control pressure and flow around thebarrier to the downstream line.

Normally, there are only two pressure zones to which the jacketcontrolled fluid may conveniently be bled, i.e., the atmosphere and thedownstream flow passage. In many installations, exhaust to theatmosphere is objectionable for a number of reasons, includingenvironmental contamination and fire hazard. The downstream passageaffords a convenient receptacle for evacuation of the jacket controlledfluid but, unfortunately, this imposes certain restrictions on theoperation of the system. Specifically, the pressure drop between thejacket and the downstream passage and hence, between the upstreampassage and the jacket, may be insufficient to insure full tubeexpansion, thus restricting the capacity of the valve. It is highlydesirable to achieve a maximum fluid flow within the range ofpermissible drop in controlled outlet pressure.

OBJECTS OF THE INVENTION It is an object of this invention to provide anexpansible tube type regulator system with a jacket bleed to a zone at apressure lower than that of the downstream passage.

It is a further object of this invention to utilize a low staticpressure zone generated by conversion of static pressure to velocitypressure.

It is a further object of this invention to provide an expansible tubetype valve under control of one pilot valve to bleed the jacket to acontained low pressure zone and a second pilot valve to bleed toatmosphere.

It is a further object of this invention to provide an expansible tubetype valve with bleed ducts to both a contained low pressure zone andthe atmosphere, with pilot valves in both ducts being selectively set tooperate sequentially, or to bleed exclusively either to the atmosphereor to the contained zone.

Other objects and advantages of this invention will become apparent fromthe description following when read in conjunction with the accompanyingdrawings.

BRIEF SUMMARY OF THE INVENTION In accordance with this invention, thepressure regulating system includes an expansible tube type valvecomprising a flexible, expansible tube stretched over a cylindrical coresleeve having consecutively, a series of upstream slots, a cylindricalbarrier blocking axial flow and a series of downstream slots. Theexpansible tube is sealed between the core and the housing at both endsso that flow is possible only by stretching the tube away from thecentral barrier, whereby upstream fluid may flow out through theupstream slots around the barrier and back in through the downstreamslots.

In a chamber or jacket around the tube, a control pressure normallyprevents expansion of the tube for flow of fluid and, conveniently, thecontrol pressure may simply be fed through from the upstream line.Hence, there is a balance of pressure between the upstream side of thecore barrier and the jacket, and flow is prevented by the hoop tensionin the sleeve stretched over the core.

A pilot valve which senses downstream pressure and opens when thedownstream pressure falls below a predetermined level opens to bleed thejacket to a zone of low pressure. It has been determined that,particularly at high flow, a zone at a static pressure lower than thatin the downstream line exists within the core sleeve on the downstreamside of the barrier at approximately the axis. This low static pressurecondition is generated by the radially inward flow of fluid convertingthe previously high static fluid pressure to a lower static fluidpressure. This invention contemplates utilization of this low staticpressure zone by providing a bleed tube which opens the outlet of thepilot valve to it, thus providing a maximum pressure drop between theupstream and jacket pressures without exhausting fluid outside of thesystem.

As a further embodiment of this invention, there is provided for usewhere atmospheric exhausts are not objectionable, a second pilot valve,also sensitive to the downstream pressure which connects the outlet ofthe first valve to the atmosphere. The provision of two pilot valvesaffords considerable flexibili ty in operation in that the second valvemay be set to operate at a slightly lower pressure to operatesequentially with and to supplement the first pilot valve.Alternatively, they may be set relative to each other so that the entirebleed is to the atmosphere, or the second pilot valve may simply be setso as to render it completely inactive in locations where atmosphericbleed is unacceptable.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a section view of anexpansible tube type valve in a pressure control system shownschematically; and

FIG. 2 is a graph depicting the performance of the system.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to FIG. 1, theexpansible tube type valve or regulator 10 employed in the pressureregulating system of this invention includes a generally cylindricalhousing 12 having an upstream hub 14 provided with a central flowpassage 16. Secured onto the downstream end of the housing 12 as bymeans of bolts 17, is a closure plate 18 carrying a downstream hub withan outlet flow passage 20 and a flange 21 for connection into thepipeline 22. A core sleeve 23 is positioned within the housing 12 as byseating within circular recesses 24 and 26 which are formed in theupstream wall of the housing 12 and the downstream closure plate 18 toprovide annular shoulders within which the opposite ends of the coresleeve 23 are retained.

The core sleeve 23 includes a dam or barrier 28 that extends completelyacross the inside of the sleeve 23 with an outer, cylindrical sealingsurface 29, to prevent flow of fluid directly therethrough, but flowaround the dam is permitted by spaced series of inlet and outlet slots30 and 32 alternating with ribs 34. The ribs 34 and central barrier 28are normally snugly embraced by an expansible tube or sleeve 40 havingan inturned flange 42 at the upstream end that is clamped and sealedbetween the core sleeve 23 and the body 12, and an outturned flange 44at the downstream end that is clamped between the body 12 and thedownstream closure plate 18.

Adjacent the upstream end of the inlet series of slots 30, the innersurface of the regulator body 12 is tapered outward at 46 to form acentral control chamber or jacket 47 defined by the larger diameter,generally cylindrical inner surface 48. A control fluid may be deliveredto the chamber through a duct 50 from a suitable source such as theupstream flow passage 16 from which the fluid P, may be tapped offthrough the fitting 51 and delivered through a variable orifice 52 tocreate a pressure drop under flow when the chamber or jacket 47 isdepressurized. Thus, in operation, the control pressure in the chamberaugments the hoop tension in the stretched resilient sleeve or tube 40normally to seal against the central barrier 28 and prevent any flow offluid around it. However, should the pressure of the control fluid inthe chamber 47 be reduced to a level wherein the upstream pressure canovercome the tube tension, the tube 40 is expanded away from the sealingsurface 29 to permit flow of fluid out through the slots 30, around thebarrier 28 into the downstream slots 32 and out the downstream passage20.

For use in gas systems, the inner surface of the housing 12 is modifiedby providing an expansion blocking surface 54 which is just slightlylarger in diameter than the expansible tube 40 and extends axially overmuch of the downstream side of the slotted sleeve core 22. That is, asubstantial portion of the expansible tube on the outlet side of theslotted sleeve is embraced by the blocking surface 54. The blockingsurface is effective to dampen the vibrations of the tube 40 that areset up by the flow of fluid around the barrier 28. This reduces theamplitude of the vibrational waves at the outlet end of the tube 40 andminimizes their effect in returning and building up trailing vibrationalwaves moving from the inlet end toward the outlet end of the tube. Withthe control fluid inlet 50 opening into the blocking surface 54, agroove 56 is provided from the control gas inlet 50 to the fullexpansion chamber 47 so as to insure the free and continuous exposure ofthe tube 40 to the control gas even though the tube 40 is expanded intofirm contact with the blocking surface 54.

Included in the fluid regulating system is a pilot valve 58, the inletof which 59 is connected to line 50 and the outlet of which is connectedto line 60, opening through port 62 in the downstream hub 21, of thevalve 10. The pilot valve is sensitive to downstream pressure P in thepipeline 22 and a duct 64, 64a exposes the downstream pressure P to adiaphragm 65 that operates the valve 58 in opposition to a spring 66,which may be adjusted at 68 to any predetermined pressure.

The structure as thus far described is in current use and its operationwill now be described. With the downstream pressure P at the desiredlevel, the expansible tube is in its sealing position shown in FIG. 1with the chamber 47 loaded by upstream pressure P, through the duct 50,51 balancing the pressure in the upstream flow passage 16. In thiscondition, the hoop tension in the tube stretched around the sealingsurface 29 is sufficient to maintain a seal and prevent flow fromupstream flow passage 16 to the downstream 20.

In the event that downstream pressure P falls below the predeterminedlevel set by the pilot valve 58, the force against the diaphragm 65 willbe overcome by spring 66 to open the pilot valve 58 and allow thechamber 47 to be evacuated through the ducts 50 and 60 to the port 62 inthe downstream flow passage 20. While this occurs, the pressure dropacross the variable orifice 52 in the duct 51, prevents pressurizationof the chamber 47 as long as there is flow in the duct 50, 59. When thedownstream pressure P reaches the desired level, the force against thediaphragm will overcome the spring 66 to close the valve. Thereafter,fluid flowing through the variable orifice 52 will gradually elevate thepressure in the chamber 47 to balance that in the upstream line 16 andallow the expansible tube 40 to contract into firm sealing engagementaround the barrier 23.

It has been determined that when the expansible tube is forced away fromthe barrier 23, the fluid flowing radially inward through the downstreamslots 32 and particularly at high flow rates, converts the staticpressure downstream of the barrier 23 into a high velocity, low staticpressure zone P, at a static pressure lower than the downstream staticpressure P Therefore, in accordance with this invention, there isprovided a bleed tube 70 connected into the port 62 to extend upstreamtherefrom on the axis of flow passage opening in a zone near the radialplane of the leading ends of the downstream slots 32. With the bleedtube so disposed, the pressure drop between the chamber 47 and the lowpressure evacuation zone P near the discharge opening 72 of the bleedtube 70 is maximized without bleeding to the atmosphere. This highpressure drop will facilitate a more effective evacuation of the chamber47 which, in turn, produces a greater pressure drop between the upstreampressure and that in the chamber 47 to enable full expansion of the tube40. This effects a concomitant increase in the rate of flow around thebarrier 28.

Referring now to FIG. 2, there is shown a graph which depicts the vastlyimproved operation with the low pressure bleed tube 70. The curve A inFIG. 2 represents the deterioration in control pressure with increase inflow rate, measure in any suitable units, such as standard cubic feedper minute (SCFM), with inlet pressure P constant. The curve B showsimproved performance with the low pressure bleed tube 70 installed. Acomparison of the curves A and B will show that at the minimum tolerableoutlet controlled pressure level (P, min.), which should be equaled orexceeded throughout operation, the flow capacity is almost doubled withthe low pressure bleeder tube installed. As a specific example, in aparticular installation an operator may tolerate no more than 15 percentpressure droop from the desired control pressure P op., and with theconventional downstream bleed, this amount of droop occurs whilerealizing a flow rate of only 6% units of fluid flow. Tests have shown,as depicted on the FIG. 2 graph, that with the low pressure bleed zonetube 70 installed, 13 units of fluid flow may be realized before thereis a 15 percent droop in outlet pressure P Referring again to FIG. 1 inareas wherein a bleed to the atmosphere is not objectionable, aconsiderable flexibility in operation is possible by addition of asecond pilot tube 74 with an outlet duct 76 open to the atmosphere andan inlet duct 78 connected to the outlet of the first pilot valve 58. Acheck valve 80 is provided in the duct 60 to prevent reverse flow fromthe bleeder tube 70. As in the first pilot valve 68, the second pilotvalve 74 senses the downstream pressure through a duct 64, 64b.

The second pilot valve 74 may be set at a pressure level slightly belowthat of the first pilot so that if the downstream pressure P, fallsbelow a level that can be compensated efficiently with the internal lowpressure bleed tube 70, the bleed may be switched to atmosphere. In suchevent, the second pilot valve 74 is also opened and the chamber 47 isevacuated through duct 50, the still open pilot valve 58, the duct 78and through pilot valve 74 to the atmospheric bleed 76.

Referring again to FIG. 2, the graph shows at the merger of curves B andC how the sequential operation of the second pilot valve bleeding toatmosphere improves performance of the system. There, the controlledpressure P follows the curve B droop until just before the minimumcontrolled pressure P min. level is reached. Then, the second pilot 74is operated and the controlled pressure P follows the curve C at anextended, substantially undiminished level until almost 18 units offluid flow are realized and then commences to droop, falling to theminimum level P min. only after achieving a flow of 18% units. Thus, thecombined internal low pressure zone bleed atmospheric bleed systems hasproduced a 200 percent improvement in fluid flow within the tolerablecontrolled pressure-droop range.

If the atmospheric bleed should be objectionable, one may merely set thesecond pilot at such a level that it will never be opened. Conversely,if exclusively atmospheric bleed is preferred, one may merely set thesecond pilot valve 74 so that it will open first. Hence, when the firstpilot valve 58 is opened at the set pressure, it will evacuate toatmosphere through the open second pilot valve 74.

While this invention has been described in conjunction with preferredembodiments thereof, it is obvious that modifications and changes may bemade by those skilled in the art without departing from the spirit andscope of the invention.

What is claimed is:

1. In an expansible tube valve comprising:

a housing having inlet and outlet flow passages,

a core sleeve secured in said housing with opposite ends thereof alignedwith said flow passages,

two axially spaced, inlet and outlet series of through openings aroundsaid core sleeve, a cylindrical barrier extending across said coresleeve intermediate said inlet and outlet series of openings, a sealingsurface around said barrier, a flexible, expansible tube having anintermediate sealing portion normally snugly embracing said sealingsurface and sealed around said housing at opposite ends thereof, and aport in said housing for introducing a control pressure fluid aroundsaid expansible sleeve, said housing being adapted for connection to apilot valve duct system which opens in response to deterioration ofpressure downstream of said outlet passage below a predetermined level,the improvement comprising:

a bleed duct adapted to connect the outlet of said pilot valve system toa zone at a pressure lower than said downstream pressure within saidcore sleeve adjacent the radial plane of the upstream portions of saidoutlet through openings. 2. The combination defined by claim 1 wherein:said outlet through openings are configurated as longitudinal slots, andsaid bleed duct opens into the interior of said core sleeve adjacent theradial plane of the upstream ends of said slots. 3. In a pressureregulating system comprising: an expansible tube valve, a pilot valveoperable by a controlled fluid pressure to open when said controlledpressure falls to a predetermined level, sensing duct means connectingsaid pilot valve to control pressure of fluid downstream of saidexpansible tube valve, said expansible tube valve comprising:

a housing having inlet and outlet flow passages, a cylindrical barrierextending partially across the interior of said housing intermediatesaid inlet and outlet flow passages, a sealing surface around saidbarrier, a flexible, expansible tube having an intermediate sealingportion normally snugly embracing said sealing surface and sealed aroundsaid housing at opposite ends thereof, a port in said housingintroducing a control pressure fluid into a chamber around saidexpansible tube, the improvement comprising:

exhaust duct means connecting said chamber to the inlet of said pilotvalve, and

bleed duct means connecting the outlet of said pilot valve to a zonewithin said housing near the downstream side of said barrier at apressure lower than said controlled fluid pressure.

4. The combination defined by claim 3 including:

a core sleeve secured in said housing with opposite open ends thereofaligned with said flow passages,

two axially spaced, inlet and outlet series of through openings aroundsaid sleeve,

said barrier extending across said sleeve intermediate said inlet andoutlet openings.

5. The pressure regulating system defined by claim 4 wherein:

said outlet through openings are configurated as longitudinal slots, and

said bleed duct opens adjacent the radial plane of the upstream ends ofsaid slots.

6. The pressure regulating system defined by claim 4 wherein:

said bleed duct opens at approximately the axis of said core sleeve.

7. The pressure regulating system defined by claim 3 includ- 6 ing:

a second pilot valve,

second sensing duct means connecting said second pilot valve to controlpressure of fluid downstream of said expansible tube valve,

a second exhaust duct connecting said bleed duct to the inlet of saidsecond pilot valve,

a check valve in said bleed duct preventing flow of fluid away from saidlower pressure zone, and

a second bleed duct connecting the outlet of said second pilot valve tothe atmosphere.

8. The pressure regulating system defined by claim 7 wherein:

said second pilot valve is adjustable to open selectively at pressurelevels higher or lower than said predetermined level.

9. The pressure regulating system defined by claim 7 wherein:

said first pilot valve is adjustable to open selectively at pressurelevels at or below said predetermined level. 10. The combination definedby claim 1 wherein: said bleed duct opens at approximately the axis ofsaid core sleeve. 11. The combination defined by claim 1 wherein: thedownstream side of said barrier is concave. 12. The combination definedby claim 3 wherein: the downstream side of said barrier is concave. 13.In a pressure regulating system comprising: an expansible tube valve, afirst pilot valve operable by a controlled fluid pressure to open whensaid controlled pressure falls to a predetermined level, first sensingduct means connecting said pilot valve to control pressure of fluiddownstream of said expansible tube valve, said expansible tube valvecomprising:

a housing having inlet and outlet flow passages, a cylindrical barrierextending partially across the interior of said housing intermediatesaid inlet and outlet flow passages, a sealing surface around saidbarrier, a flexible, expansible tube having an intermediate sealingportion normally snugly embracing said sealing surface and sealed aroundsaid housing at opposite ends thereof, a port in said housingintroducing a control pressure fluid into a chamber around saidexpansible tube, the improvement comprising:

exhaust duct means connecting said chamber to the inlet of said firstpilot valve,

first bleed duct means connecting the outlet of said first pilot valveto a low pressure zone downstream of said barrier,

a second pilot valve,

second sensing duct means connecting said second pilot valve to controlpressure of fluid downstream of said expansible tube valve,

a second exhaust duct connecting said bleed duct to the inlet of saidsecond pilot valve,

a check valve in said bleed duct preventing flow of fluid away from saidlow pressure zone, and

a second bleed duct connecting the outlet of said second pilot valve tothe atmosphere.

14. The pressure regulating system defined by claim 13 wherein:

said second pilot valve is adjustable to open selectively at pressurelevels higher or lower than said predetermined level.

15. The pressure regulating system defined by claim 13 wherein:

sure levels at or below said predetermined level.

1. In an expansible tube valve comprising: a housing having inlet andoutlet flow passages, a core sleeve secured in said housing withopposite ends thereof aligned with said flow passages, two axiallyspaced, inlet and outlet series of through openings around said coresleeve, a cylindrical barrier extending across said core sleeveintermediate said inlet and outlet series of openings, a sealing surfacearound said barrier, a flexible, expansible tube having an intermediatesealing portion normally snugly embracing said sealing surface andsealed around said housing at opposite ends thereof, and a port in saidhousing for introducing a control pressure fluid around said expansiblesleeve, said housing being adapted for connection to a pilot valve ductsystem which opens in response to deterioration of pressure downstreamof said outlet passage below a predetermined level, the improvementcomprising: a bleed duct adapted to connect the outlet of said pilotvalve system to a zone at a pressure lower than said downstream pressurewithin said core sleeve adjacent the radial plane of the upstreamportions of said outlet through openings.
 2. The combination defined byclaim 1 wherein: said outlet through openings are configurated aslongitudinal slots, and said bleed duct opens into the interior of saidcore sleeve adjacent the radial plane of the upstream ends of saidslots.
 3. In a pressure regulating system comprising: an expansible tubevalve, a pilot valve operable by a controlled fluid pressure to openwhen said controlled pressure falls to a predetermined level, sensingduct means connecting said pilot valve to control pressure of fluiddownstream of said expansible tube valve, said expansible tube valvecomprising: a housing having inlet anD outlet flow passages, acylindrical barrier extending partially across the interior of saidhousing intermediate said inlet and outlet flow passages, a sealingsurface around said barrier, a flexible, expansible tube having anintermediate sealing portion normally snugly embracing said sealingsurface and sealed around said housing at opposite ends thereof, a portin said housing introducing a control pressure fluid into a chamberaround said expansible tube, the improvement comprising: exhaust ductmeans connecting said chamber to the inlet of said pilot valve, andbleed duct means connecting the outlet of said pilot valve to a zonewithin said housing near the downstream side of said barrier at apressure lower than said controlled fluid pressure.
 4. The combinationdefined by claim 3 including: a core sleeve secured in said housing withopposite open ends thereof aligned with said flow passages, two axiallyspaced, inlet and outlet series of through openings around said sleeve,said barrier extending across said sleeve intermediate said inlet andoutlet openings.
 5. The pressure regulating system defined by claim 4wherein: said outlet through openings are configurated as longitudinalslots, and said bleed duct opens adjacent the radial plane of theupstream ends of said slots.
 6. The pressure regulating system definedby claim 4 wherein: said bleed duct opens at approximately the axis ofsaid core sleeve.
 7. The pressure regulating system defined by claim 3including: a second pilot valve, second sensing duct means connectingsaid second pilot valve to control pressure of fluid downstream of saidexpansible tube valve, a second exhaust duct connecting said bleed ductto the inlet of said second pilot valve, a check valve in said bleedduct preventing flow of fluid away from said lower pressure zone, and asecond bleed duct connecting the outlet of said second pilot valve tothe atmosphere.
 8. The pressure regulating system defined by claim 7wherein: said second pilot valve is adjustable to open selectively atpressure levels higher or lower than said predetermined level.
 9. Thepressure regulating system defined by claim 7 wherein: said first pilotvalve is adjustable to open selectively at pressure levels at or belowsaid predetermined level.
 10. The combination defined by claim 1wherein: said bleed duct opens at approximately the axis of said coresleeve.
 11. The combination defined by claim 1 wherein: the downstreamside of said barrier is concave.
 12. The combination defined by claim 3wherein: the downstream side of said barrier is concave.
 13. In apressure regulating system comprising: an expansible tube valve, a firstpilot valve operable by a controlled fluid pressure to open when saidcontrolled pressure falls to a predetermined level, first sensing ductmeans connecting said pilot valve to control pressure of fluiddownstream of said expansible tube valve, said expansible tube valvecomprising: a housing having inlet and outlet flow passages, acylindrical barrier extending partially across the interior of saidhousing intermediate said inlet and outlet flow passages, a sealingsurface around said barrier, a flexible, expansible tube having anintermediate sealing portion normally snugly embracing said sealingsurface and sealed around said housing at opposite ends thereof, a portin said housing introducing a control pressure fluid into a chamberaround said expansible tube, the improvement comprising: exhaust ductmeans connecting said chamber to the inlet of said first pilot valve,first bleed duct means connecting the outlet of said first pilot valveto a low pressure zone downstream of said barrier, a second pilot valve,second sensing duct means connecting said second pilot valve to controlpressure of fluid downstream of saiD expansible tube valve, a secondexhaust duct connecting said bleed duct to the inlet of said secondpilot valve, a check valve in said bleed duct preventing flow of fluidaway from said low pressure zone, and a second bleed duct connecting theoutlet of said second pilot valve to the atmosphere.
 14. The pressureregulating system defined by claim 13 wherein: said second pilot valveis adjustable to open selectively at pressure levels higher or lowerthan said predetermined level.
 15. The pressure regulating systemdefined by claim 13 wherein: said first pilot valve is adjustable toopen selectively at pressure levels at or below said predeterminedlevel.